expression-purification-liposome-binding-budding-yeast-snx-bar

Expression, Purification, and Liposome Binding of Budding Yeast SNX-BAR Heterodimers

SNX-BAR proteins are an evolutionarily conserved class of membrane remodeling proteins that play key roles in sorting and trafficking of protein and ...

Yale School of Medicine

The yeast SNX4 sub-family of sorting nexin containing a Bin-Amphiphysin-Rvs domain (SNX-BAR) proteins, Snx4/Atg24, Snx41 and Atg20/Snx42, are required for endocytic recycling and selective autophagy. Here, we show that Snx4 forms 2 functionally distinct heterodimers: Snx4-Atg20 and Snx4-Snx41. Each heterodimer coats an endosome-derived tubule that mediates retrograde sorting of distinct cargo; the v-SNARE, Snc1, is a cargo of the Snx4-Atg20 pathway, and Snx4-Snx41 mediates retrograde sorting of Atg27, an integral membrane protein implicated in selective autophagy. Live cell imaging of individual endosomes shows that Snx4 and the Vps5-Vps17 retromer SNX-BAR heterodimer operate concurrently on a maturing endosome. Consistent with this, the yeast dynamin family protein, Vps1, which was previously shown to promote fission of retromer-coated tubules, promotes fission of Snx4-Atg20 coated tubules. The results indicate that the yeast SNX-BAR proteins coat 3 distinct types of endosome-derived carriers that mediate endosome-to-Golgi retrograde trafficking.

Distinct complexes of yeast Snx4 family SNX-BARs mediate retrograde trafficking of Snc1 and Atg27.

Lipids and proteins mix it up in Philly.

Cargo-selective and nonselective autophagy pathways employ a common core autophagy machinery that directs biogenesis of an autophagosome that eventually fuses with the lysosome to mediate turnover of macromolecules. In yeast ( Saccharomyces cerevisiae) cells, several selective autophagy pathways fail in cells lacking the dimeric Snx4/Atg24 and Atg20/Snx42 sorting nexins containing a BAR domain (SNX-BARs), which function as coat proteins of endosome-derived retrograde transport carriers. It is unclear whether endosomal sorting by Snx4 proteins contributes to autophagy. Cells lacking Snx4 display a deficiency in starvation induced, nonselective autophagy that is severely exacerbated by ablation of mitochondrial phosphatidylethanolamine synthesis. Under these conditions, phosphatidylserine accumulates in the membranes of the endosome and vacuole, autophagy intermediates accumulate within the cytoplasm, and homotypic vacuole fusion is impaired. The Snx4-Atg20 dimer displays preference for binding and remodeling of phosphatidylserine-containing membrane in vitro, suggesting that Snx4-Atg20-coated carriers export phosphatidylserine-rich membrane from the endosome. Autophagy and vacuole fusion are restored by increasing phosphatidylethanolamine biosynthesis via alternative pathways, indicating that retrograde sorting by the Snx4 family sorting nexins maintains glycerophospholipid homeostasis required for autophagy and fusion competence of the vacuole membrane.

Lipid trafficking by yeast Snx4 family SNX-BAR proteins promotes autophagy and vacuole membrane fusion.

Synaptobrevin/vesicle-associated membrane protein 2 (VAMP2) is an essential soluble -ethyl maleimide-sensitive factor attachment protein receptor (SNARE) protein that has been extensively studied in its role in synaptic vesicle fusion. However, sorting and trafficking of VAMP2 within the endosomal system is not well understood. Here, we use the yeast VAMP2 homologue Snc1 to investigate the pathways and signals required for endocytic trafficking. We identify two genetically distinct retrieval pathways from the endosomal system: a plasma membrane recycling pathway that requires the Rcy1 F-box protein and a retrograde pathway originating from the multivesicular/prevacuole endosome dependent on the Snx4-Atg20 sorting nexin complex. Lysine residues within the transmembrane domain of Snc1 are necessary for presentation of a Snx4-Atg20-dependent sorting signal located within its juxtamembrane region. Mutations of the transmembrane lysine residues ablate retrograde sorting and subject Snc1 to quality control via sorting into the degradative multivesicular endosome pathway. Degradative sorting requires lysine residues in the juxtamembrane region of Snc1 and is mediated by the Rsp5 ubiquitin ligase and its transmembrane adapters, Ear1 and Ssh4, which localize to endosome and vacuole membranes. This study shows that Snc1 is trafficked between the endosomal system and the Golgi apparatus via multiple pathways and provides evidence for protein quality control surveillance of a SNARE protein in the endo-vacuolar system.

Retrograde trafficking and quality control of yeast synaptobrevin, Snc1, are conferred by its transmembrane domain.

The endosomal system functions as a network of protein and lipid sorting stations that receives molecules from endocytic and secretory pathways and directs them to the lysosome for degradation, or exports them from the endosome via retrograde trafficking or plasma membrane recycling pathways. Retrograde trafficking pathways describe endosome-to-Golgi transport while plasma membrane recycling pathways describe trafficking routes that return endocytosed molecules to the plasma membrane. These pathways are crucial for lysosome biogenesis, nutrient acquisition and homeostasis and for the physiological functions of many types of specialized cells. Retrograde and recycling sorting machineries of eukaryotic cells were identified chiefly through genetic screens using the budding yeast Saccharomyces cerevisiae system and discovered to be highly conserved in structures and functions. In this review, we discuss advances regarding retrograde trafficking and recycling pathways, including new discoveries that challenge existing ideas about the organization of the endosomal system, as well as how these pathways intersect with cellular homeostasis pathways.

Article	Year
Distinct complexes of yeast Snx4 family SNX-BARs mediate retrograde trafficking of Snc1 and Atg27. Traffic (Copenhagen, Denmark)\| PubMed ID: 28026081	2017
Lipids and proteins mix it up in Philly. Molecular biology of the cell\| PubMed ID: 29535177	2018
Lipid trafficking by yeast Snx4 family SNX-BAR proteins promotes autophagy and vacuole membrane fusion. Molecular biology of the cell\| PubMed ID: 29949447	2018
Retrograde trafficking and quality control of yeast synaptobrevin, Snc1, are conferred by its transmembrane domain. Molecular biology of the cell\| PubMed ID: 31067149	2019
Retrograde trafficking and plasma membrane recycling pathways of the budding yeast Saccharomyces cerevisiae. Traffic (Copenhagen, Denmark)\| PubMed ID: 31471931	2019

Mengxiao Ma

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